nghttpx: Don't add 0-length DATA when response HEADERS bears END_STREAM flag

src/shrpx_downstream.cc

@@ -740,7 +740,8 @@ bool Downstream::get_expect_final_response() const {
 }
 
 bool Downstream::expect_response_body() const {
-  return http2::expect_response_body(req_.method, resp_.http_status);
+  return !resp_.headers_only &&
+         http2::expect_response_body(req_.method, resp_.http_status);
 }
 
 namespace {

src/shrpx_downstream.h

@@ -185,7 +185,8 @@ struct Response {
         http_status(0),
         http_major(1),
         http_minor(1),
-        connection_close(false) {}
+        connection_close(false),
+        headers_only(false) {}
 
   void consume(size_t len) {
     assert(unconsumed_body_length >= len);
@@ -202,6 +203,10 @@ struct Response {
   unsigned int http_status;
   int http_major, http_minor;
   bool connection_close;
+  // true if response only consists of HEADERS, and it bears
+  // END_STREAM.  This is used to tell Http2Upstream that it can send
+  // response with single HEADERS with END_STREAM flag only.
+  bool headers_only;
 };
 
 class Downstream {

src/shrpx_http2_session.cc

@@ -1024,6 +1024,10 @@ int on_response_headers(Http2Session *http2session, Downstream *downstream,
     }
   }
 
+  if (frame->hd.flags & NGHTTP2_FLAG_END_STREAM) {
+    resp.headers_only = true;
+  }
+
   rv = upstream->on_downstream_header_complete(downstream);
   if (rv != 0) {
     // Handling early return (in other words, response was hijacked by